In a normal spine, a healthy intervertebral disc can compress about 1 mm when subjected to an axial load of 750 pounds force (3.4 kN). This ability to compress in response to axial loads provides the spine with an effective shock absorber against high loads.
The leading cause of lower back pain arises from rupture or degeneration of lumbar intervertebral discs. Pain in the lower extremities is caused by the compression of spinal nerve roots by a bulging disc, while lower back pain is caused by collapse of the disc and by the adverse effects of articulation weight through a damaged, unstable vertebral joint. One proposed method of managing these problems is to remove the problematic disc and replace it with a prosthetic disc that allows for the natural motion between the adjacent vertebrae (“a motion disc”).
Many prosthetic motion discs seek to mimic the flexion, extension and lateral bending of the natural disc by providing an articulating interface. For example, U.S. Pat. No. 6,368,350 (“Erickson”) discloses a three-piece articulating motion disc providing two articulation surfaces. However, in order to minimize the amount of wear debris associated with the articulating interface, articulating motion discs tend to be made of very hard materials. Accoirdingly, articulating motion discs do not provide the shock absorbing function of a natural intervertebral disc.
Other motion discs seek to provide the shock absorbing function of the natural disc by providing a cushion within the prosthetic disc. For example, U.S. Pat. No. 5,824,094 (“Serhan”) discloses a prosthetic motion disc comprising a rubber core member sandwiched between two metal endplates.
Another example of a cushion-type disc is disclosed in U.S. Pat. No. 4,911,718 (“Lee”). The Lee patent discloses a motion disc having a central core. See Lee at col. 3, lines 51–64. The core component of the Lee design seeks to mimic the nucleus pulposus of a natural disc, while the laminae seek to mimic the annulus fibrosus component of a natural disc, and the endplate components provide for anchorage to the adjacent endplates of the adjacent vertebral bodies.
In every design disclosed by Lee, the motion disc has a pair of endplates; and the outer laminae component comprises fibers.
Although the recognition in the Lee device of the need to mimic both the nucleus pulposus and annulus fibrosus components has its advantages, the provision in the Lee device of prosthetic endplates is problematic because it is known that the shear forces associated with natural vertebral movement will cause large shear stresses at the interfaces between the prosthetic endplates (on one hand) and the prosthetic nucleus pulposus and annulus fibrosus components (on the other hand).
Like the Lee patent, U.S. Pat. No. 5,545,229 (“Parsons”) also discloses a motion disc having a pair of endplates, and intermediate components comprising an inner core and an outer ring. However, the outer ring of Parsons is designed so as to have a higher hardness than the inner core component. In particular, Parsons discloses that the outer ring is made of a “stiffer elastomeric material surrounding said central core to approximate the size and shape of a natural annulus fibrosus. See Parsons at col. 4, lines 7–16. Parsons specifically discloses preferred embodiments in which the core component has a hardness of between 20 and 70 shore-A, while the outer ring has a hardness of between 40 and 80 shore-A. Parsons further discloses that the device preferably uses the same elastomeric material in various degrees of stiffness for its various components (col. 4, lines 51–53). Table II discloses one particular embodiment in which the core is made of a block copolymer having a hardness of 35 shore-A, the outer ring is made of the same material having a hardness of 70 shore-A, the endplates are made of the same material having a hardness of 90 shore-A.
In every design disclosed by Parsons, the motion disc has a pair of endplates; and the outer ring is preferably no more than 80 shore-A.
The endplates are specifically designed to provide a mechanical interlock with the adjacent bone surface.
Although Parsons improves upon Lee by providing for a higher stiffness in the annulus fibrosus component, Parsons nonetheless retains the problematic endplate components of Lee.
Published U.S. patent application No. 2003/0023311 (“Trieu”) discloses an artificial disc implant that is resistant to migration in and/or expulsion from an intervertebral disc space. In one form, the implant includes a load bearing elastic body surrounded in the disc space by an anchoring, preferably resorbable, outer shell. In certain forms, the elastic body is surrounded by a supporting member, such as a band or jacket, and the supporting member is surrounded by the outer shell. In another form of the invention, an implant is provided that has locking features and optional shape memory characteristics. In yet another aspect, nucleus pulposus implants are provided that have shape memory characteristics and are configured to allow short-term manual, or other deformation without permanent deformation, cracks, tears, breakage or other damage.